Orography-driven chemical denudation in the Lesser Antilles: Evidence for a new feed-back mechanism stabilizing atmospheric CO2

Abstract

In this paper we present chemical composition data for major elements in rivers from three islands of the Lesser Antilles. The Lesser Antilles are a tropical volcanic subduction arc and are characterized by steep gradients of relief, bedrock age and precipitation. They constitute a natural laboratory where the response of the weathering engine to large variations of runoff can be understood. Data indicate that the Lesser Antilles are characterized by extremely variable chemical weathering (40-430 t/km 2 /a) and CO 2 consumption (300-3500.10 3 mol/km 2 /a) rates, amongst the highest found on Earth and consistent with the previous studies on the weathering of volcanic rock. A noteworthy observation is that, along the runoff gradient, concentrations of rock-derived solutes do not follow a pure dilution law and that a buffering mechanism exists stabilizing solute concentrations. As a result concentrations vary much less than runoff and chemical weathering rates are mainly controlled by runoff. Precipitation patterns in the Lesser Antilles are essentially orographic and controlled by the adiabatic decompression of the water-saturated Atlantic air masses. The production of acidity by volcanic degassing is an additional factor that modulates the runoff effect. Two main conclusions can be drawn from this study. First, chemical weathering fluxes of oceanic islands are strongly dependent upon relief repartition, which cautions the use of regional mean values to compare volcanic islands. Second, volcanic activity in the Lesser Antilles subduction arc, by creating relief, promotes high orographic precipitation and/or infiltration regimes, that in turn results in elevated chemical weathering and atmospheric CO 2 consumption fluxes. This feedback mechanism, implying mainly precipitation and relief, is proposed to act in complement to the temperature-related feedback proposed by previous authors for stabilizing the atmospheric CO 2 content of the atmosphere in response to volcanic CO 2 degassing. This study highlights the importance of the water cycle in controlling chemical weathering of volcanic arc islands and associated CO 2 consumption rates.

Evaluation of controls on silicate weathering in tropical mountainous rivers: Insights from the Isthmus of Panama

“…1). This finding supports the role of orographic-driven precipitation on elevated weathering rates observed in Guadeloupe (Gaillardet et al, 2011). The importance of runoff on weathering rates has been well documented through worldwide river compilation studies and modeling studies (Meybeck, 1987;Gaillardet et al, 1999).…”

“…This linkage of physical and chemical weathering rates was reinforced by subsequent studies at active plate margins (Jacobson and Blum, 2003;Lyons et al, 2005;Gaillardet et al, 2011), where the majority of the physical erosion on Earth's surface currently occurs. It has been hypothesized that high physical erosion rates continuously provide abundant mineral surfaces and/or maintain an ideal soil thickness for uninterrupted chemical weathering (West, 2012).…”

Short Communication: Aging of basalt volcanic systems and decreasing CO<sub>2</sub> consumption by weathering

“…The factors that module these fluxes are a subject to uncertainty. and have an elevated topography, a combination, which can cause elevated runoff due to an 58 orographic effect (Gaillardet et al, 2011). However, the here suggested aging effect on 59 weathering rates from a volcanic system has not been evaluated.…”

Geomorphic regime modulates hydrologic control of chemical weathering in the Andes–Amazon

“…The geodynamic processes that build relief typically also drive erosion (e.g., Montgomery and Brandon, 2002), and relief may control fluid transit times by creating longer fluid flow paths (McGuire et al, 2005), though evidence for this relationship remains equivocal (Tetzlaff et al, 2009). If fluid transit times and topographic length scales are positively correlated, then mountainous regions may be efficient chemical weathering systems because they have relatively long fluid transit times, short mineral residence times, and large water fluxes driven by orographic precipitation (Gaillardet et al, 2011;Li et al, 2014;Maher and Chamberlain, 2014). These interactions may also control the processes fractionating stable isotopes of dissolved elements (Bouchez et al, 2013), such that understanding solute chemistry may unlock clues for interpreting the geologic record of variations in seawater isotopic composition (e.g., the Li isotope record; Misra and Froelich, 2012).…”